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Solid-State Balanced Microphone Preamplifier
Solid-State Balanced Microphone Preamplifier
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Old 15th June 2019, 06:17 AM   #81
spladski is offline spladski  United Kingdom
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There are two options. There are PFETs that will work, including TH FETs but the choice becomes limited. The integration of the SMD pad is virtually no work. The FET gate position is in the same place as the base. The FET can be mounted on the underside of the board if necessary. If the FET is symmetrical then it can be mounted topside.

Let's call your version the P version and my one the N version. Doesn't the PCB work for both if you swap the power rails and reverse the decoupling caps? That was the advantage of removing the opamps. I would hope you got more than one proto PCB made.
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Old 15th June 2019, 05:45 PM   #82
H713 is offline H713  United States
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Yes, I ordered five of the first PCB version. I used JLCPCB, and the boards seem to be very high quality. By this point I've changed out a lot of parts in the one I've populated, and so far the pads have held up extremely well.

I'm presently working on adding the SMD pads. One question, however, is what the 22R resistors (R14 and R15) are doing in your spice simulation.

I think that if I'm clever about this I can make one PCB work for both.
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Old 15th June 2019, 08:18 PM   #83
spladski is offline spladski  United Kingdom
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Yes, precisely. The additional resistors might help with the jump in gain issue. I don't know. Try it. I hope you are sitting comfortably.

The basic concept is an amp gain block. Anything that an opamp can do, this can do. So visualize an 12 pin module. The transistors are connected. 10 connections are for the collectors and emitters. 2 connections for input. Since the pin-outs of SMD P and N devices are the same, the module can be populated for P and N versions of the circuit.
So now this creates options for creating a range of modules which can be referred to as 'differential amp module' DAM. If the composite TH/SMD pad is used for the input pair, there is total flexibility.
1. A signal module using the smallest components.
2. Low power module where the output pair are SOT223

Further options are to use the matched FETs like the LSK489 for the input, generating two more modules. And so on.

Supplying the PCBs to DIYers would probably create an open source hive industry. The modules can spawn many projects:
Mic preamp
Microphone with remote gain control and compressor. That would be a first.
Guitar preamp
Headphone amp
Electrostatic headphone amp
Phono head amp, including one that lives in the pickup arm which the vinyl types are after.
Power amp driver
etc.

You should give it a name.
Now you can have a cup of tea/coffee and mull it over.

Last edited by spladski; 15th June 2019 at 08:42 PM. Reason: Correction
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Old 15th June 2019, 08:38 PM   #84
spladski is offline spladski  United Kingdom
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I am also wondering if a combination of P and N modules could be used for distortion cancellation. Another brain melt project.

Last edited by spladski; 15th June 2019 at 08:41 PM.
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Old 15th June 2019, 08:57 PM   #85
H713 is offline H713  United States
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Updated version of the board is done- All I did was add an SMD footprint for the input transistors. Most of the work in this had to do with correcting issues with the pad switch, adding mounting holes, and adding a couple extra ground pads for the transformer shields.
Attached Files
File Type: zip H713 Microphone Preamplifier V2.zip (73.4 KB, 14 views)
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Old 16th June 2019, 05:25 AM   #86
spladski is offline spladski  United Kingdom
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That should work. The alternative is to use a triangle for the BJT pads and place the SMD in the middle, with the pads touching. It takes up slightly less space. I don't see any decoupling for PSU rails. You should have decoupling as with any opamp. 1206 SMD 10uF 25V can be used in combination with 0.1uF which can be TH. These can be multi-layer ceramic so they don't need to be reversed for either P or N version. Add a separate connector for the +48V. Then place the PCB power connector on either side of the PCB depending on the P or N version. If the connector is keyed, it prevents blowing it up by accident. A right angle connector might be better. Or devise some other scheme.

Last edited by spladski; 16th June 2019 at 05:47 AM. Reason: More info
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Old 16th June 2019, 06:07 AM   #87
spladski is offline spladski  United Kingdom
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The SMD is tracked on one side. Not an issue with 2SK3557 as it is symmetrical. Might want to track it both sides.
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Old 16th June 2019, 08:51 AM   #88
spladski is offline spladski  United Kingdom
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This is the color/distortion control implemented in the least disruptive way. The input is switched to single ended. So it can be implemented with a two pole changeover switch. Might make a good fuzz box at the extremes. Try on the prototype.
Attached Files
File Type: asc H713_FET_2SK3557_color_1.asc (6.0 KB, 6 views)
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Old 16th June 2019, 08:56 AM   #89
H713 is offline H713  United States
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Note that the "connectors" for the power, input and transformers are really just through-holes where wires can be soldered to. I've never been a huge fan of PCB mount XLRs, and there is no standardized pinout (or footprint) for PCB mount transformers. I figure this will be mounted in a chassis of some form anyhow.

Good call on the PSU decoupling. Something a little more substantial than a .1uF is probably in order.
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Old 17th June 2019, 10:38 AM   #90
spladski is offline spladski  United Kingdom
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This one is less aggressive, less interactive with gain and doesn't affect noise.
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File Type: asc H713_FET_2SK3557_color_2.asc (6.1 KB, 5 views)
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